In many instances of the manufacturing of electronic apparatuses, the mechanical aspects of manufacturing are automatically controlled. Especially in recent years, such automatic controls are required to be extremely precise. In magnetic disk apparatuses and optical disk apparatuses for instance, the positioning of their heads used for the recording and reproduction of information calls for a very high level of accuracy because the track pitch is reduced to increase the storage capacity.
In such an electronic apparatus, a controller is provided to regulate its mechanical operations, and this controller generates electric power to drive the mechanical parts (hereinafter collectively referred to as the "plant"). The role of the controller generally is to keep physical quantities (the travelling speed of the arm, for example) at their respectively set values. Therefore, in order to ensure the accuracy of control, the physical quantities of the plant should be monitored. If the physical quantities to be controlled in the plant could be directly monitored and fed back to the plant by electric signals or the like, there would be no problem, but such monitoring usually is difficult, and in many cases the physical quantities can be detected only indirectly through sensors or the like. For this reason, an estimator is used to estimate which cannot be directly detected physical quantities in the plant. General consideration of such an estimator is described in Gene F. Franklin et al., Digital Control of Dynamic Systems, 2nd ed., Chapter 6.3, pp. 250-264. One of the examples of the use of an estimator in a specific plant is disclosed in the U.S. Pat. No. 4,766,967, where the estimator is used in a motor vehicle.
In detecting physical quantities from information detected by an estimator, however, the physical quantities cannot be accurately estimated if the detected information contains noise irrelevant to them. An object of the present invention is to provide an estimator which can restrain the impact of such noise.